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Weird 34970A - Labview behavior

Question asked by maheshh on Aug 29, 2008
First of all I am not very sure whether the behavior that I am witnessing is a function of Labview environment/ 34970A meter / the mechanical fixturing of the DUT. But here it goes - and I have been trying to tackle this issue for 4 months now:

The attached picture shows the schematic of the setup and how it is designed/intended to work.
Step 1:

Every time I put a test PCB in, and collect voltages for 1000 test points I should be done.

But everytime I start the data collection, I get about 15 - 25 % out of the 1000 voltage readings as garbage.

Garbage voltage could mean - intermittent or open at the pogo pin mechanical interface. So I run step 2 which is 2-wire resistance to establish whether there are any continuity issues.
Step 2:

Remember - 2-wire measurement will send a small test current through each of the 1000 test points - which means each of the pogo pin will see some current pass thruogh it.

I do not see any continuity issues.

Then "without disturbing the setup" I re-run the voltage recording and magically all the 100% data is good.

Step 3:

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My question: why this behavior? If step 1 and step 3 are identical, why is there about 15-25% garbage data in step 1 and a perfect data in step 3? Will the small test current in step 2 (during 2-wire measurement) help in establishing a good electrical connection at the pogo-pin interface?

Thanks in advance.  

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